Production of rustless iron



Patented Nov. 26, 1935 UNITED STATES PATENT OFFICE 2,021,919 PRODUCTIONor aUs'rLEss IRON of Maryland No Drawing.

Application July 20, 1933,

Serial No. 681,372

6 Claims.

This invention relates to corrosion resistant irons and steels and moreparticularly to an art of producing the same.

Among the objects of my invention is the simple, eflicient andeconomical production of rustless irons and steels of a desirable finegrain size and of improved physical characteristics, made to a desiredspecification of iron, carbon, chromium and nitrogen, either with orwithout supplementary elements such as nickel, copper, cobalt, titanium,molybdenum, tungsten, vanadium and the like. i

The invention accordingly consists in the combination of elements,composition of ingredients and mixture of materials, and in the severalsteps and the relation of each of the same to one or more of the othersas described herein, and the scope of the application of which isindicated in the following claims.

As conducive to a clever understanding of certain features of myinvention it may be noted at this point that, as more particularlypointed out in my co-pending application, Serial No. 645,637, entitledNon-corrosive alloy, filed December 3, 1932, many highly beneficialcharacter istics are given rustless iron and steel by adding to themetal a small percentage of nitrogen. The

irons and steels containing nitrogen are of an inherently fine, even,grain structure;. they are of improved workability over a wider range ofworking temperatures; they are less subject to grain growth,brittleness, decarburization and fatigue; and they are more durable andof higher impact values than heretofore known alloys of the classindicated.

One of the outstanding objects of my invention is the eilicient,economical and thoroughly practical production of rustless iron andsteel, either with or without supplementary additions indicated above,wherein a desired nitrogen content is reliably and economicallyachieved. In the practice of my invention a suitable furnace, forexample, an electric arc furnace, is charged with desired raw materials,such as rustless iron scrap and/or ordinary low-carbon steel scrap andan oxidizing agent suchas iron ore, roll scale orthe like. forming abath of ferrous metal covered by an oxidizing slag, and then brought toa uniformly high temperature. Chromium additions are made either alongwith the initial melt down ingredients, using, for example, high carbonferrochrome or chrome ore, or these additions may be made to the bathand slag formed as indicated above, using chrome ore.

This charge is melted down Certain practical advantages, such as savingsin power consumption, improved furnace operating conditions, andthelike, are gained by making the chromium additionsboth along with theinitial charge of ingredients and to the bath of 5 metal and slag aftermelt down.

Under the action of the strongly oxidizing slag and the hightemperatures employed, carbon coming from the furnace electrodes and,furnace atmosphere, as well as from the raw materials, 10 is effectivelyexcluded and/or removed from the bath of ferrous metal. Along with theoxidation of carbon there is an incidental oxidation of chromium fromthe metal; thechromium oxides formed being. transferred into the slag.After complete melt-down of the ingredients to form a ferrous metal bathof low carbon content covered by a slag containing the oxides ofiron'and chromium is achieved, as indicated above, a'suitable reducingagent, such as ferrosilicon, is charged into the furnace to recover ironand chromium from the slag overlying the bath of metal to enable theproduction of rustless iron in an efficient and economical manner. Alongwith the reducing agent there is added a desired quantity of basic slagforming materials, such as burnt lime to assure basic conditions duringthe reduction of the oxides contained in the slag and a consequentminimum attack of the acid constituents formed thereby on the furnacelining.

Under the action of the reducing agent employed the oxides of iron andchromium contained in the slag are reduced and the nascent iron andchromium resulting from this reduction step gravitate from the slag intothe underlying bath of metal. I

To give. finished metal of a desired nitrogen content, the reduction ofthe oxides of iron and chromium contained in the slag is carried out inthe immediate presence of a nitrogen containing 0 gas. For this purposecommercial dry bottled nitrogen may be conveniently employed, thenitrogen being added to the slag by maintaining a nitrogen atmosphereunder slight pressure within the furnace during the reduction period orby directly introducing nitrogen gas into the slag .at one or morepoints to assure intimate contact between the nascent metals and thenitrogen gas.

The iron and chromium coming from the slag, in their nascent states,absorb nitrogen and carry this nitrogen either in a free or combinedstate into the bath of ferrous metal. The nitrogen atmosphere ismaintained in and about the slag during the reduction period until adesired quantity of nitrogen gas is added to the bath of metal. 5

The character and duration of the nitrogen atmosphere, or the amount ofnitrogen containing gas and the manner of introducing this gas into theslag in order to achieve a desired nitrogen content in the finishedrustless iron are features which are determined empirically. I

The heat of metal is refined and finished in accordance with standardpractice after which the metal is poured into suitable molds and allowedto cool. The finished metal is of fine even grain structure and may beworked over a wider range of temperatures than heretofore known rustlessirons and steels. The metal is less subject to grain growth,brittleness, decarburization and fatigue and is more durable and ofhigher impact values than alloy irons of the class indicated, all asmore particularly pointed out in my copending application referred toabove.

As illustrative of the practice 'of my invention a 6-ton Heroultelectric arc furnace, having graphite electrodes and rated three-phase,25 cycle, 1500 KVA at 110 to 180 volts, is first prepared for thereception of a charge by arcing on electrode butts to heat up thefurnace. The furnace is preferably provided with a chromite brick bottomwhich is carried up to a height somewhat above the slag line of thefurnace. For reasons of economy, a lining containing chrome ore ispreferably rammed in over the chromite brick hearth lining; sodiumsilicate being used as a convenient binder. and roof are convenientlylined with silica brick.

After the preheating, as indicated above, the furnace is,illustratively, charged with 12,600 pounds of rustless iron scrap,analyzing about 17% chromium and about .10% carbon; 3250 pounds ofordinary low-carbon steel scrap; 2850 pounds of chrome ore, analyzingabout 48% chromium oxide (Cr-20s) and about 19% iron oxide (FeO); and1,000 pounds of roll scale which is substantially 100% magnetic ironoxide.

Electric power is applied to the furnace and the charge of ingredientsis rapidly melted down to form a bath of molten iron containing chromiumwith a small percentage of carbon, and an overlying slagv containing theoxides of iron and chromium. With the continued application of power,the temperature of the bath of molten metal and the supernatant slag isbrought up to a point considerably higher than that ordinarily employedin usual steel melting practice in an electric furnace.

While no reliable method is known for precisely determining thetemperature of the metal bath beneath the slag blanket, it is estimatedthat this temperature, designated as a temperature of superheat, isapproximately 3000 F. to 3200 E, which is some 100 F. to 300 F. higherthan the temperature ordinarily employed in electric steel meltingpractice. At this temperature of superheat the oxidizing slag is moreactive in combining with carbon contained in the metal bath and carboncoming from the furnace atmosphere, to effectively remove and/or excludecarbon from the metal bath.

Incidental to the oxidation of carbon, there is an oxidation of chromiumfrom the bath, the chromium oxide entering the slag. The extent of theloss of chromium into the slag is minimized by the rapidity of theinitial melt-down and bringing the bath and slag to the elevatedtemperature of superheat indicated above.

Under the strongly oxidizing action of the slag overlying the metal baththe carbon conmnt is. continuously lowered. When tests on samples Thefurnace side wallstaken from the bath indicate a desired low carboncontent is reached (about .05%) the melt-down and oxidizing stage is atan end.

In order to effect a recovery of the metals contained in the slag asoxides of iron and chro- 5 mium, there is added, illustratively, 1400pounds of crushed 75% ferrosilicon and 4500 pounds of hot dry burnt limeas rapidly as furnace conditions permit. The additions of ferrosiliconand lime-fuse and become incorporated in the slag 10 andeffect areduction of the oxides of iron and chromium contained therein,producing metal, which goes into the underlying bath of molten metal andvarious silicates which remain in the slag and tend to render the slagacid. A basic 15 character is obtained by the introduction of the largequantities of lime indicated above.

In order to efiiciently and economically achieve a desired nitrogencontent in the tapped metal, nitrogen gas is introduced into the slagover- 20 lying the metal bath immediately after the addition offerrosilicon and lime is commenced. During the reduction of the oxidesof iron and chromium contained in the slag, nascent iron and chromiummetals form, and gravitate 25 through the slag and into the underlyingbath of metal. These metals in their nascent state actively absorbnitrogen and carry this nitrogen into the bath of molten metal.

The nitrogen gas is introduced directly into 80 the slag within thoseregions where the reduction of the oxides contained in the slag is mostactively going on, ordinarily within the regions of the slag immediatelyadjacent the electrodes.

For the proportions of the ingredients illus- 86 tratively set forthabove, a nitrogen content in the finished metal of about .05% isattained by passing a gentle flow of nitrogen gas into the slag througha inch pipe for a period of from 10 to 15 minutes. Commercial bottleddry nitrogen is an available and relatively inexpensive source ofnitrogen gas which lends itself to an ease of operation and precisecontrol.

As the lime and ferrosilicon are added to the slag the oxides containedtherein are progressive- 1y reduced and the color of the slag changesfrom black to a light green. The reducing action is accompanied by atransfer of the metal from the slag to the underlying bath of molteniron and chromium. When the colorof the slag indicates thatsubstantially all of the oxide content has been reduced, this slag ispreferably completely removed from the surface of the metal bath and 'abasic finishing slag of lime, ferrosilicon, and fluorspar or like flux,is formed in accordance with standard practice.

Final-additions of lump low-carbon ferrosilicon and low-carbonferromanganese are added to adjust the analysis of the bath to thedesired specifications of silicon and manganese. Additions of thesupplementary alloying elements, nickel, copper, cobalt, titanium,tungsten, vanadim, and the like are made as desired.

The heat of metal-is then tapped into suitable molds and allowed tocool. The tapped metal, for the embodiment illustratively set forthabove, weighs 17,300 pounds and analyzes about .08% carbon, 17%chromium, .06% nitrogen, .40% manganese, .35% silicon, with the desiredsupplementary alloy additions indicated above, with the usual lowpercentages of sulphur and phosphorus, and the balance substantiallyiron.

Thus it will be seen that there has been provided in this invention anart in which the various objects hereinbefore noted together with manythoroughly practical advantages are successfully achieved. It will beseen that the process of producing rustless irons and steels of adesired nitrogen content is simple, direct, and economical and that adesired nitrogen content may be efiiciently and economically attainedwith a minimum of eflort and/ or expensive equipment.

While in the embodiment of my invention 11- lustratively set forth abovea bath of ferrous metal containing chromium covered by a slag containingoxides of iron and chromium is prepared by melting down a charge ofrustless iron scrap, ordinaryv low-carbon steel scrap, chrome ore andmill scale (and the oxide content of this slag is reduced in thepresence of nitrogen to eflect a recovery of iron and chromium and addnitrogen to the bath), it will be understood that any desired rawmaterials such as ordinary scrap and/or rustless iron scrap, high-carbonferrochrome and iron oxide may be melted down to form a bath of metalcontaining iron and chromium covered by a supernatant slag containingoxides of iron and chromium and these oxides reduced in the immediatepresence of nitrogen to effect a recovery of the metals, andgive adesired nitrogen content to the metal, where such procedure is founddesirable.

Likewise, while in the embodiment illustratively set forth above, all ofthe chrome ore is added along with the initial charge of ingredients itwill be understood that, where furnace operating conditions warrant it,a portion of the chrome ore may be added after the initial melt-down oreven along with the reducing agent, ferrosilicon, and lime.

'Although best results in the introduction of nitrogen are achieved bydirectly adding dry nitrogen to those regions of the slag where thereduction operation is most actively carried out, good results areachieved where nitrogen is forced into the slag from above, as bymaintaining a nitrogen containing atmosphere within the furnaceimmediately above the slag at a slight pressure.

As many possible embodiments may be made of my invention and as manychanges in the embodiment hereinbefore set forth it will be under-'-stood that all matter described herein is to be interpreted asillustrative, and not in a limiting sense.

I claim:

1. In the production of nitrogen-containing rustless iron and steel offine grain structure, the art which includes, preparing a bath of molteniron and an overlying slag containing. oxides of chromium, reducingtheoxides of chromium contained in said. slag thereby enriching saidbath,=and adding nitrogen to said slag during said reducing operationwhereby considerable quantities of nitrogen are absorbed by the nasasaid bath.

3. In the production of nitrogen-containing rustless iron and steel offine grain structure, the 16 art, which includes, preparing a bath ofmolten iron and an overlying slag containing oxides of iron andchromium, reducing said oxides thereby enriching said bath, and passingnitrogen containing gas into the slag during said reducing 20 operationwhereby nitrogen is absorbed by the 4 iron and chromium coming from saidslag and entering the bath of metal, thereby supplying appreciablequantities of nitrogen to said bath.

4. In the production of nitrogen-containing 25 rustless iron and steelof flne grain structure, the art which includes, preparing a bath ofmolten iron; adding chrome ore and a reducing agent to said bath wherebythe ore is reduced, the chromium entering the bath; and supplyingnitrogen 30 to said ore during the reduction thereof, the nitrogen beingabsorbed by the nascent chromium, whereby nitrogen is added to saidbath.

5. In the production of nitrogen-containing rustless iron and steel'offine grain structure, the art which includes, melting down a charge ofrustless iron scrap, chrome ore and iron oxide to form a bath of metalcovered by an oxidizing slag; maintaining said bath and slag at hightemperature thereby oxidizing carbon from the 40 bath together withchromium, the chromium oxide' entering the slag; and while reducing theoxides in said slag bubbling nitrogen gas into the slag, the resultingmetals absorbing nitrogen and thereby adding nitrogen to said bath along45 with the contributions from the slag. I

6. In the production of nitrogen-containing rustless iron and steel offine grain structure, the art which includes, preparing a bath of molteniron and an overlying slag containing oxides of so chromium, reducingthe oxides of chromium contained in the slag under strongly basicconditions thereby enriching said bath, and adding nitrogen to said slagduring said basic reduction period 55 whereby considerable quantities ofnitrogen are absorbed by the nascent chromium, thereby supplyingappreciable quantities of nitrogen to said bath.

WILLIAM BELL ARNESS. m

